1. Field of the Invention
The present invention generally relates to the chemical arts. More particularly, the present invention relates to an apparatus and method for generating hydrogen gas by decomposing or reforming a liquid fuel.
2. General Background and State of the Art
The growing popularity of portable electronic devices has produced an increased demand for compact and correspondingly portable electrical power sources to energize these devices. Developments in robotics and other emerging technology applications are further increasing the demand for small, independent power sources.
At present, storage or rechargeable batteries are typically used to provide independent electrical power sources for portable devices. However, the amount of energy that can be stored in storage or rechargeable batteries is insufficient to meet the need of certain applications.
Hydrogen/air fuel cells (H/AFCs) have enormous potential as a replacement for batteries. Because they can operate on very energy-dense fuels, fuel cell-based power supplies offer high energy-to-weight ratios compared with even state-of-the-art batteries. Fuel cells are of particular interest to the military, where significant efforts are being made to reduce the weight of power supplies that soldiers must carry to support high-tech, field-portable equipment. There is also considerable potential for utilizing fuel cell-based power supplies for commercial applications, particularly where small size and low weight are desirable.
A common H/AFC is a polymer electrolyte membrane (PEM) fuel cell. PEM fuel cells are constructed of an anode and a cathode separated by a polymer electrolyte membrane.
Functionally, fuel cells generate electricity by reacting hydrogen with oxygen to produce water. Since oxygen can typically be obtained from the ambient atmosphere, only a source of hydrogen must be provided to operate a fuel cell. Merely providing compressed hydrogen is not always a viable option, because of the substantial volume that even a highly compressed gas occupies. Liquid hydrogen, which occupies less volume, is a cryogenic liquid, and a significant amount of energy is required to maintain the extremely low temperatures required to maintain it as a liquid. Furthermore, there are safety issues involved with the handling and storage of hydrogen in the compressed gas form or in the liquid form.
Several alternative approaches are available. These alternatives include ammonia decomposition and hydrocarbon reformation. Ammonia decomposition is relatively easy. Ammonia can be thermo-catalytically cracked at relatively low temperatures to produce a gas mixture that is 75% hydrogen by volume. Hydrocarbon fuels are somewhat more technically challenging, because hydrocarbon reformation requires relatively higher temperatures, and the simple cracking of hydrocarbons produces a solid residue which is undesirable in a fuel cell application. However, the reformation of hydrocarbon fuels offers the incentive of enabling a higher energy density fuel to be used, as compared with the use of ammonia as a fuel source, i.e., the production of a greater mass of hydrogen per unit mass of fuel. Consequently, there is a desideratum for an apparatus that has the flexibility to effectively and efficiently generate hydrogen from either ammonia or hydrocarbon fuel.
The ammonia decomposition reaction can be represented as follows:2NH3→N2+2H2  (1)
The simple hydrocarbon cracking reaction can be represented as follows:CnH(2n+2)→Cn(solid)+(n+1)H2  (2)
The formation of solid residues can be avoided through the use of oxidative cracking processes or by employing steam reforming. Oxidative cracking be represented as follows:CnH(2n+2)nO2→nCO2+(n+1)H2  (3)
Steam reforming can be represented as follows:CnH(2n+2)2nH2O→nCO2+(3n+1)H2  (4)
It is a drawback of ammonia decomposition that traces of un-reacted ammonia (typically <2000 ppm) remain in the product gas stream. One of the challenges of utilizing ammonia to produce hydrogen for a fuel cell is that H/AFCs do not tolerate ammonia in the hydrogen feed gas, so the trace amounts of ammonia in the hydrogen produced by an ammonia cracker must be removed before the remaining H2/N2 mixture is supplied to a fuel cell.
It is a drawback of hydrocarbon reformulation that the actual product is a mixed gas stream that contains substantial amounts of carbon monoxide (CO). Furthermore, the product is a gas stream that also contains partially oxidized hydrocarbons. Both carbon dioxide and partially oxidized hydrocarbons can poison the anode electro-catalysts used in PEM fuel cells. Thus, utilizing either ammonia decomposition, oxidative cracking or steam reforming requires additional steps to purify the hydrogen, or decompose the impurities. Such additional processes add size, cost, and complexity to a hydrogen generation system, making achieving a compact, low cost, and portable system more difficult. Therefore, it is also a desideratum to provide a hydrogen generation system that can be used to provide hydrogen to a fuel cell, which requires minimal or no additional processing to purify the hydrogen that is produced before such hydrogen can be used in a fuel cell.
To compete with battery-based power supplies, such an H/AFC apparatus needs to be compact and reliable. It is a further desideratum to develop a portable hydrogen supply with a volume less than 1 liter and a mass less than 1 kg that can produces up to 50 watts of electrical power, with a total energy output of 1 kWh. Commercially available metal hydride storage cylinders are available in 920 gm cylinders that contain the equivalent of 100 W-h of hydrogen;
thus, a total energy output of 1 kWh represents an order of magnitude increase in energy density over commercially available apparatuses.